1. Field Of The Invention
This invention relates to electrochemical gas detectors and, more particularly, to a three-electrode electrochemical sensor having a compact design.
2. Background Art
The theory and general operation of three-electrode electrochemical sensors used in toxic or other gas detector systems are well known. These sensors, also referred to as cells, typically include a sensing electrode, a reference electrode spaced from the sensing electrode by a porous separating mat containing an aqueous-based electrolyte contacting the sensing and reference electrodes through the mat, and a counter electrode also separated from the reference electrode by a porous mat containing the electrolyte, and a gas diffusion limiting means. The sensing electrode can be a porous, gas diffusion electrode having a coating of a catalytic material on the surface adjacent to the electrolyte. The gas to be sensed, referred to as the object gas, diffuses either alone or in combination with other gases through the diffusion limiting means to the sensing electrode. The object gas undergoes a reaction, either a reduction or an oxidation, at the interface of the electrolyte and the catalytic material on the sensing electrode. The catalytic material is selected to promote the reaction with a particular object gas, but to catalyze little or none of the other gases that may accompany the object gas.
The reference electrode is used in conjunction with an electronic circuit to maintain a predetermined potential difference between the sensing and reference electrodes. This potential difference also is selected so as to encourage the desired oxidation or reduction reaction of the object gas at the sensing electrode. This potential difference also is selected so that other undesirable reactions will be suppressed as much as possible and, thereby, will not interfere with the desired reaction resulting from the presence of the object gas. The predetermined potential difference between the reference electrode and the sensing electrode is maintained by the electronic circuit without drawing current from the reference electrode. Within limits, this potential difference does not affect the magnitude of the current generated by the reaction of the object gas at the sensing electrode. The magnitude of the current is controlled by the diffusion limiting means.
The result of either the oxidation or reduction reaction at the sensing electrode is the production of ions and electrons. These charged ions migrate through the electrolyte to the counter electrode. A conductive wire or other conduction path is connected external of the cell between the sensing and counter electrodes to complete the electrical path, to allow electrons to flow between the counter and sensing electrodes, and to permit another electrochemical reaction to take place at the counter electrode. With all other conditions remaining constant, such as temperature, gas pressure, and humidity, the number of electrons generated by the reaction at the sensing electrode will be directly proportional to the amount of object gas diffusing to the sensing electrode. The electronic current flowing through the external circuit between the sensing and counter electrodes can be measured by an ammeter or the like and provide a quantitative reading of the concentration of object gas present.
Prior art three-electrode electrochemical sensors useful for detecting an object gas in an atmosphere are shown, for example, in U.S. Pat. Nos. Re. 31,914, Re. 31,915 and Re. 31,916. A two-electrode sensor is shown in U.S. Pat. No. 3,755,125. However, these sensors are rather bulky due to the positioning of the electrodes with respect to each other and because they require an electrolyte reservoir in the cell.
It is, accordingly, a first object of the present invention to provide a three-electrode electrochemical sensor for gas detection which has a compact design.
The prior art has provided numerous designs of three-electrode electrochemical sensors of compact design, as shown, for example, in U.S. Pat. Nos. 4,406,770, 4,521,290, 4,633,704 and 4,769,122, and U.K. Patent Application No. 2 140 566. See also U.S. Pat. Nos. 3,950,980, 4,025,412, 4,132,616, and 4,587,003.
The reference electrodes used in three-electrode electrochemical sensors are conveniently of the air type. While the steady-state air-electrode potential depends on variables such as gas pressure, temperature, and electrolyte composition, it is desirable that the reference potential be stable. However, fluctuations from this potential do occur in normal operation. Such fluctuations affect the accuracy of the measurements developed by a sensor, render the sensor less immune to interference from other gases, and cause the sensor's operation to vary over its life. This deviation in the air reference electrode's redox potential from the theoretical value is often characterized as an instability in the reference electrode potential. The stability problems with air reference electrodes are reduced or minimized in three-electrode electrochemical sensors when the overall air reference electrode is large in size. Problems have been observed when the reference electrode is reduced to the small sizes used in compact designs. In such compact electrochemical sensors, the reference electrode is often not sufficiently stable and reproducible to provide accurate, dependable and consistent gas measurements.
Therefore, it is a second object of the present invention to provide a compact, three-electrode electrochemical sensor for gas detection which includes a large and stable air reference electrode that can operate reliably at the air-electrode potential.